1. Field of the Invention
The present invention relates to a disk reproducing apparatus and method and, more particularly, to a device and method of automatically detecting the kinds of disk mounted in an apparatus for reproducing a plurality of kinds of disks.
Generally, one representative type of optical disk performing non-contact reading by using a laser beam is a compact disk (hereinafter, referred to as a CD). However, for the disk recording/reproducing apparatus, higher speed, overwriting and higher density (larger capacity) are increasingly required. Meanwhile, for the digital video compression technology, MPEG (Moving Picture Experts Group) has become more practical in use. Accordingly, digital moving picture disk media like a digital video disk (DVD) will emerge as the core media of multimedia storage.
2. Description of the Related Art
FIG. 1 is a block diagram of a conventional DVD player. In this figure, disk 20 is a high-density optical disk, which can be DVD in this case. Pickup unit 111 is a shortwave optical head. It is assumed that the numerical aperture be 0.6, and that the red semiconductor laser wavelength be used. Signal amplifier 112 amplifies a light signal output from pickup unit 111 after converting it into an electrical signal. Demodulator/ECC decoder 113 demodulates the modulated signal output from signal amplifier 112, and corrects the error produced during playback. Servo controller 114 receives the output (focusing&tracking error signal) of signal amplifier 112 and the output (constant linear velocity control signal) of demodulator/ECC decoder 113 in order to control pickup unit 111. System decoder 116 decodes the DVD data stream output from demodulator/ECC decoder 113, the stream being divided into a video (elementary) data stream and a audio (elementary) data stream. Video decoder 117 decodes the video data stream output from system decoder 116 so that it is converted into the form of reproducible video data. Encoder 118 encodes the video data output from video decoder 117 into a corresponding video displaying mode (NTSC or PAL), and then outputs them to a display. Audio decoder 119 converts the audio data stream encoded and output from system decoder 116 into the original data. D/A converter 120 converts the data output from audio decoder 119 into an analog audio signal. In FIG. 1 the system controller for controlling the overall operation of the disk reproducing apparatus is not shown.
The DVD player constructed as above uses modulation, error correction and a video encoding different from those of a general CD player so that their signal processings are completely different. The pit size and track pitch of a DVD and a CD are different, and their pickup unit 111 laser wavelength and the numerical aperture of the objective lens are different.
In the DVD recording/reproducing apparatus, it is important to reduce the size of a focusing spot for the purpose of higher density. For this, firstly, the laser wavelength becomes shorter, secondly, the objective lens' numerical aperture becomes larger, and thirdly, a modulation having an excellent encoding efficiency is used. Here, for a DVD laser wavelength is 650-635 nm of a red semiconductor laser, the objective lens' numerical aperture is 0.6, and EFM-plus (Eight to Fourteen Modulation plus) is used for modulation.
In order to reproduce information from the high-density optical disk, pickup unit 111 should be able to form a small focusing spot. The diameter of the focusing spot is proportional to the wavelength .lambda. of light, and inversely proportional to the objective lens' numerical aperture, as shown in FIG. 2. At present, the lower limit of the semiconductor laser wavelength is about 635 nm, as described above. Generally speaking, the DVD uses a laser wavelength of 650 nm as its standard, and the CD uses a laser wavelength of 780 nm. The DVD's numerical aperture is 0.6, and the CD's is 0.45 so that the DVD's diameter of focusing spot becomes over about 60% of the CD's. Therefore, the DVD can obtain a playback signal equal to the CD's at about 2.6 times the recording density of the CD. The DVD improves the waveform of the playback signal through the actual circuit processing so that it has a recording density about 4.2 times the CD's.
FIG. 3 shows the relationship between pits and optical focus with respect to a DVD and CD. FIG. 3 shows various dimensions of and area ratios between the DVD and the CD.
The characteristics of a DVD and a CD are shown in the following table 1.
TABLE 1 ______________________________________ CD DVD REMARKS ______________________________________ DISK DIAMETER 120 nm 120 nm DVD is a both- sided multi- DISK THICKNESS 1.2 nm 0.6 nm layer. RECORDING CAPACITY 640 Mbyte 4.7 Gbyte In case of single-sided single layer LASER WAVELENGTH 780 nm 650 nm OBJECTIVE'S 0.45 0.6 NUMERICAL APERTURE MINIMUM PIT SIZE 0.9 .mu.M 0.4 .mu.M TRACK PITCH 1.6 .mu.M 0.74 .mu.M MODULATION MODE EFM EFM-plus ERROR CORRECTING CIRC RS-PC MODE ______________________________________
In the table 1, the CD and DVD of the same disk diameter have many differences. However, their compatibility is very important because the CD format has an enormous amount of soft resources. The primary impedance in the compatibility between a DVD and a CD is to use a separate optical pickup. In other words, in order to reproduce information on a disk, part of the optical system is formed according to the thickness of disk. For this reason, the objective lens is designed in accordance with the thickness of disk. If the thickness of the disk deviates from the value designed, spherical aberration is produced, and the convergence of optical focus is decreased. More specifically, a DVD is 0.6 mm-thick, whereas a CD is 1.2 mm-thick so that the two kinds of disks cannot be reproduced with a single pickup unit. For their compatibility, separate optical pickup units must be used.
In order to overcome such a problem, a double-focus optical unit was suggested for both a CD and a DVD. In the optical pickup unit shown in FIG. 4, the optical focus for a DVD and the optical focus for a CD are formed spaced apart by a predetermined distance in the direction of an optical axis. This solves the problem caused by the difference of thickness of disk. The principle of the double-focus optical pickup shown in FIG. 4 is to overlap the operations of two kinds of lenses by using a hologram. The numerical aperture of the objective lens of the optical pickup is 0.6 in accordance with the standards of a DVD, and a hologram is provided to diffract part of its central light as if a lens refracts it. The light not diffracted by the hologram is converged by the objective lens whose numerical aperture is 0.6, to thereby form the optical focus of a DVD. The diffracted light forms a hologram to be suitable for 0.4 numerical aperture of the hologram and objective lens and 1.2 mm-thick CD.
In case of the optical pickup shown in FIG. 4, the optical focus for a CD is formed farther than that for a DVD. Here, the photodetector is disposed so that the reflection image of the optical focus converged on the disk is formed thereon. If the DVD is reproduced, the optical focus of the DVD is converged on the disk, and the optical focus for the CD deviates so that the image becomes unclear. The image of the optical focus for the DVD is formed on the photodetector but the reflection light at the optical focus for the CD is scattered widely to give no external impacts in DVD's playback. The reverse operation is performed in CD's playback.
In case of a system having a pickup capable of reproducing both DVD and CD or pickups capable of independently reproducing DVD and CD, it should be operated in a corresponding mode by detecting the kind of disk inserted. The discrimination of DVD and CD can be implemented in various ways: to compare the sizes of the focus error detection signals reproduced by the optical pickup; or to compare the sizes of RF signals in accordance with the pit size.
However, the aforesaid method has the disadvantages that a long time is required to make a distinction between a CD and a DVD, and accuracy is deteriorated.